The overall objective is to determine the intraovarian and extraovarian endocrine relationships which govern function during pregnancy in the rabbit and rat. The luteotropic effects of estradiol formed by aromatization within the rat corpus luteum will be studied with respect to the ability of intraluteal estradiol to maintain luteal structure and luteal progesterone synthesis. The role of the theca interna cell of the corpus luteum will be investigated in the rat by selectively removing these cells prior to autotransplantation of preovulatory follicles beneath the kidney capsule where they luteinize. As indication of normal function, we will determine the ability of such corpora lutea to synthesize progesterone and estradiol in amounts required for the maintenance of pregnancy. Finally, the possibility that the rabbit blastocyst produces an LH-like hormone, which stimulates the corpus luteum prior to implantation, will be studied by careful measurements of serum progesterone in pregnant and pseudopregnant (control) rabbits matched for similar numbers of corpora lutea. BIBLIOGRAPHIC REFERENCES: Holt, J.A., W.F. Heise, S.M. Wilson and P.L. Keyes.: Lack of gonadotropic activity in the rabbit blastocyst prior to implantation. Endocrinology 98:904-909, 1976. Elbaum, D.J., and P.L. Keyes.: Synthesis of 17B-Estradiol by isolated ovarian tissues of the pregnant rat: aromatization in the corpus luteum. Endocrinology 99:573-579, 1976.